The present invention relates to the field of lighting devices, and more particularly, to a ballast for a gas discharge lamp.
Gas discharge lamps are widely used for general illumination and offer substantial advantages such as efficiency, color, coolness and shape over incandescent lamps. Gas discharge lamps include fluorescent lamps and high-intensity discharge (HID) lamps. These lamps are driven with a ballast. The ballast provides a predetermined level of current to the lamp which causes the lamp to emit light. To initiate current flow through a gas discharge lamp, the ballast provides a relatively high start-up voltage. After the gas discharge lamp has been ignited, a lower operating voltage is applied.
A conventional ballast generally provides predetermined operating parameters for characteristics adapted for a single lamp type. Operating parameters include a start-up voltage, a preheat time with a preheat frequency or pulse width which sets a preheat current, an operating frequency and a frequency ramping profile. The frequency ramping profile shifts the operating frequency from preheat to ignition, and then to operating. For example, a 40 watt gas discharge lamp may require a start-up voltage of 800 volts, whereas the start-up voltage for a 40 watt gas discharge lamp will be different.
However, gas discharge lamps of different wattages generally have different operating parameters. For example, the operating parameters for a 20 watt gas discharge lamp are different than those for the 40 watt gas discharge lamp. Consequently, the gas discharge lamp is generally ignited with a high enough start-up voltage that will support the desired lamp type and other lamp types having a start-up voltage less than the desired lamp type. The other operating parameters supporting the desired lamp type will also generally support these other lamp types requiring a lower startup voltage.
An advantage of this approach is in terms of manufacturing cost since a single ballast can be used instead of providing multiple versions of gas discharge lighting devices, each with a uniquely configured ballast. However, to support these different lamp types, the same high start-up voltage is applied to all gas discharge lamps even if a lower start-up voltage is better suited.
An excess voltage applied to a gas discharge lamp may decrease the life of the lamp. This difference in usable lamp life may be especially important in applications where the gas discharge lamp is turned on and off on a regular basis, such as in storage areas and spaces with occupancy sensors.
In view of the foregoing background, it is an object of the present invention to provide a ballast and associated method that is compatible with different types of gas discharge lamps.
This and other objects, features and advantages in accordance with the present invention are provided by a ballast comprising a power supply, and a controller connected to the power supply. The controller preferably comprises a memory having a plurality of desired operating parameters stored therein for respective different types of gas discharge lamps, and a sensing circuit for causing the power supply to supply a current to the gas discharge lamp prior to start-up and sensing a voltage based thereon indicative of a type of the gas discharge lamp.
The ballast preferably further comprises a control circuit for causing the power supply to provide the desired operating parameters based upon the type of gas discharge lamp. Since the desired operating parameters are applied to the gas discharge lamp, the life of the lamp is increased. The ballast according to the present invention is thus compatible with different types of gas discharge lamps, such as lamps of different wattages.
The desired operating parameters may include at least one of a start-up voltage, preheat time and a preheat frequency, an operating frequency, a frequency ramping profile which shifts the operating frequency from preheat to ignition to operation, fault detection levels, and minimum and maximum dimming frequency to be used with an external dimming control.
The gas discharge lamp preferably comprises a housing, at least one electrode carried by the housing, and a gas contained within the housing and contacting the at least one electrode. In one embodiment of the present invention, the sensing circuit senses the voltage across one of the electrodes.
The sensing circuit may include a switching circuit connected to a first voltage reference and to the electrode. The control circuit, which may include a microcontroller, provides a control signal for operating the switching circuit so that the current is supplied to the electrode. In one embodiment of the present invention, the switching circuit comprises at least one photocoupler. The sensing circuit may further include a sense resistor connected between the electrode and a second voltage reference.
The sensed voltage may be either across the electrode alone or across the electrode and the sense resistor. The sensed voltage is converted to a digital value by an analog to digital converter, which may be internal to the microcontroller, for example. The sensing circuit senses the voltage prior to every start-up. The sensed voltage is compared to a database of lamp type voltages stored within the memory. If the sensed voltage is within a particular range, then the control circuit causes the power supply to provide the desired operating parameters based upon the voltages corresponding to the stored lamp type voltage.
In yet another embodiment of the ballast according to the present invention, the controller preferably comprises a fault detection circuit connected between the gas discharge lamp and the control circuit. A fault counter within the control circuit counts the number of times the ballast has had a fault or has failed to ignite. This information may then be used to modify the start-up characteristics of the ballast prior to attempting to restart the ballast again. In addition, fault information may be transferred to a master controller or computer external the gas discharge lighting device.
Another aspect of the invention relates to a method for operating a ballast compatible with different types of gas discharge lamps. The method preferably comprises storing a plurality of desired operating parameters for respective different types of gas discharge lamps. A current is supplied to the gas discharge lamp via a power supply prior to start-up and a voltage based thereon indicative of a type of the gas discharge lamp is sensed. The method preferably further includes controlling the power supply to provide the desired operating parameters based upon the type of gas discharge lamp.
The controlling preferably comprises comparing the sensed voltage to a plurality of lamp type voltages corresponding to respective different types of gas discharge lamps, and selecting the desired operating parameters based upon the sensed voltage corresponding to a stored lamp type voltage. The gas discharge lamp comprises at least one electrode, and the sensing comprises sensing the voltage across the at least one electrode.